With the increased use of solar energy heating, it becomes almost an absolute necessity that thermal energy be stored in order to spread the excess solar heat available during the daytime for use at night and on cloudy days. The use of heat of fusion material for this heat storage has gained increasing favor in recent times due to its low cost and high heat of fusion per unit weight. Such heat of fusion materials desirably should meet the criteria of low cost, availability in large quantities, and simplicity of preparation. In addition, it is preferred that they be non-toxic, non-flammable, non-combustible and non-corrosive. The lowest cost materials for use are large volume chemicals based on compounds of sodium, potassium, magnesium, aluminum and iron. Preferably, the materials are in the form of salt-hydrates and their eutectics. The type of low cost compounds are restricted to chlorides, nitrates, sulfates, phosphates and carbonates, while additives or modifiers may include borates, hydroxides and silicates. Among those low cost salt-hydrates having high heat of fushion, low cost and lowest incompatibility due to undesirable properties are included:
__________________________________________________________________________ Heat of Fusion Chemical Melting BTU per Density Compound Point, .degree. F Pound lb/ft.sup.3 __________________________________________________________________________ Calcium chloride hexahydrate CaCl.sub.2.sup.. 6H.sub.2 O 84-102 75 102 Sodium carbonate decahydrate Na.sub.2 CO.sub.3.sup.. 10H.sub.2 O 90-97 106 90 Disodium phosphate dodecahydrate Na.sub.2 HPO.sub.4.sup.. 12H.sub.2 O 97 114 95 Calcium nitrate tetrahydrate Ca(NO.sub.3).sub.2.sup.. 4H.sub.2 O 102-108 60 114 Sodium sulfate decahydrate Na.sub.2 SO.sub.4.sup.. 10H.sub.2 O 88-90 108 97 Sodium thiosulfate pentahydrate Na.sub.2 S.sub.2 O.sub.3.sup.. 5H.sub.2 O 118-120 90 104 __________________________________________________________________________
In use these materials are usually placed in sealed containers together with a nucleating agent and subjected by the system to successive heating and cooling cycles above and below the melting point of the heat of fusion material selected to use the "stored" heat or cold.
The need for nucleating agents is described in U.S. Pat. No. 2,677,664 issued May 4, 1954 to Maria Telkes. As is described in the Telkes patent, a suitable heterogenous nucleating agent may be borax (sodium tetraborate decahydrate) in small quantities, about 2 to 5%. These nucleating agents provide for the necessary seeding to initiate the formation of crystals and thereby avoid supercooling which can occur in liquid solutions at rest. Other known nucleating techniques may be used to promote crystallization. Crystallization, of course, is necessary to make use of the heat of fusion of the material. With supercooling, only the specific heat of the material is used. The specific heat of a material is far less than its heat of fusion -- hence the need for nucleation. When using sodium tetraborate decahydrate (a near-isomorphous nucleating agent) in combination with Na.sub.2 SO.sub.4.sup.. 10H.sub.2 O, it is possible to obtain complete crystallization in a melt by inverting or occasionally shaking the container after the crystals start to form. When used for the storage of heat energy, unfortunately, it is not always convenient, or for that matter, possible to shake the containers.
Another problem encountered in utilizing heat of fusion materials has been that after several cycles of heating and cooling, liquid tends to separate from the salt-hydrate and form salt crystals of lower hydration or for that matter, anhydrous salt, with a corresponding loss of available heat of fusion.
Stated differently, the melting of sodium sulfate decahydrate and many other salt-hydrates is found to be partly incongruent. That is, during melting some anhydrous sodium sulfate remains undissolved in its water of crystallization which is released in the melting. Due to its higher density, sodium sulfate sinks in the saturated solution. When the mixture solidifies again without mechanical mixing or stirring, dissolved sodium sulfate combines with water of crystallization, but those heavy crystals of sodium sulfate on or near the bottom of the container recombine only with water molecules in their immediate vicinity forming solid sodium sulfate decahydrate crystals. This solid layer prevents further recombination of the remaining sodium sulfate with the balance of the water of crystallization. Due to this effect, molten sulfate decahydrate, when it solidifies without stirring or without additives, forms three distinct layers -- a bottom layer of white anhydrous sodium sulfate crystals, some embedded into crystals of sodium sulfate decahydrate, then a larger intermediate layer of translucent sodium sulfate decahydrate crystals, and on top, a layer of liquid saturated solution. The heat of fusion required to melt this salt is 108 BTU's per pound which could be released again if the salt could be homogenized during solidification by stirring or by suitable additives. During cooling, (without homogenizing or stirring), the heat release is less, because part of the sediment cannot regain its water of crystallization. Some saturated solution remains in this case when the mixture is cooled depending upon the solubility of the salt. Separation and settling of the salt-hydrate must be prevented.
Over the years various thickening agents have been included in heat storage mixtures as additives with thhe aim of producing a gel in which the salt-hydrate does not settle out even over successive heating/cooling cycles. Many different thickening agents have been tried including wood shavings, wood pulp, sawdust, various types of cellulosic mixtures, and an organic material sold under the tradename "METHOCELL", starch and organic alginates. Inorganic thickening agents were also used, such as silica gel, diatomaceous earth, and other finely divided silica products. Many of these materials perform quite well but only for a limited number of cycles. Some of the organic materials become slowly hydrolyzed or decomposed by bacteria or by enzyme action. In many cases such action can be prevented or slowed by adding small quantities of formaldehyde or other suitable agents. Wood shavings, wood pulp and the like were not found to be durable enough. Silica gel formed in the mixture itself proved to be a hindrance to filling the mixture into containers because it thickened too quickly.
Eutectics of the salt-hydrates are used to modify the freezing point of the various hydrates. For the most part, the eutectics are based on low cost compounds such as sodium chloride, ammonium chloride, potassium chloride and other known types. Most eutectics also require a nucleating agent as well as a homogenizing or thickening agent since they tend to melt partly incongruently. The homogenizing agent prevents the settling of the higher density anhydrous components.
Accordingly, it is an object of this invention to obviate many of the disadvantages of the prior art heat of fusion mixtures.
Another object of this invention is to provide an improved heat of fusion material in which water and the salt-hydrate have a reduced tendency to separate during freezing and melting.
A further object of this invention is to provide an improved method for making heat of fusion mixtures which melt the same way as congruent materials do.